Combined GPS positioning system and communications system utilizing shared circuitry

ABSTRACT

A combined GPS and communication system having shared circuitry. The combined system includes an antenna for receiving data representative of GPS signals, a frequency converter coupled to the antenna, a frequency synthesizer coupled to the frequency converter, an analog to digital converter coupled to the frequency converter and a processor coupled to the frequency converter. The processor processes the data representative of GPS signals to determine a pseudorange based on the data representative of GPS signals. The integrated communication receiver includes a shared component which is at least one of the antenna, the frequency converter, the frequency synthesizer and the analog to digital converter. Typically, in certain embodiments the processor also demodulates communication signals received as well as controls the modulation of data to be transmitted as a communication signal through a communication link.

This application is a continuation of application Ser. No. 08/652,833,filed May 23, 1996, which is now U.S. Pat. No. 6,002,363.

BACKGROUND OF THE INVENTION

This application is a continuation in part of three patent applicationsfiled by the same inventor, Normal F. Krasner, on Mar. 8, 1996, thesethree applications being: An Improved GPS Receiver and Method forProcessing GPS Signals (Ser. No. 08/612,669) now U.S. Pat. No.5,663,734; An Improved GPS Receiver Utilizing a Communication Link (Ser.No. 08/612,582) now U.S. Pat. No. 5,874,914; An Improved GPS ReceiverHaving Power Management (Ser. No. 08/613,966).

FIELD OF THE INVENTION

The present invention relates to receivers which perform the dualfunctions of (a) determining their position by means of reception oftransmission from global positioning satellite systems and (b)performing communications to and from other locations in order toreceive position location commands and other information and transmit inreturn data representative of position information which was foundthrough reception of transmissions from global positioning satellitesystems.

BACKGROUND ART

The combination of GPS systems and other communications systems isreceiving considerable interest, especially in the areas of personal andproperty tracking. An example of such a combination is shown in U.S.Pat. No. 5,225,842. The communication link allows an inexpensive GPS(Global Positioning System) receiver located on a mobile person orobject (for example, a vehicle or animal) to transmit its accuratelydetermined position to remote locations which monitor this activity.Applications of this technology include security, truck fleet tracking,emergency response, inventory control, etc. The prior art has performedsuch combinations by mating separate GPS receivers and communicationsystems using suitable electronic interfaces between the two, forexample, serial communication ports, etc. Common housings and a commonpower supply are often shared in order to reduce over all cost.Nevertheless, the prior art provides systems utilizing separatecircuitry to perform the GPS and communication functions.

It has heretofore been impractical to combine much of the circuitry ofthe two different systems since all known GPS receivers utilizespecialized hardware, called "correlators", to process the receivedsignals from a multiplicity of satellites. This specialized hardwarediffers markedly from that used in communications receivers andtransceivers such as cellular telephones and pagers. In manycommunication receivers, such as those found in cellular telephones andpagers, signal processing functions are performed using general purposedigital signal processing integrated circuits, such as the TMS 320family from Texas Instruments. Accordingly, the signal processinghardware of the two different systems are incompatible in a combined GPSand communication system.

SUMMARY OF THE INVENTION

The present invention provides a combined apparatus which is typically amobile system having a GPS receiver and an integrated communicationreceiver. In a typical embodiment, the system comprises the GPS receiverwhich includes a GPS antenna for receiving data representative of GPSsignals and includes a processor, such as a digital processor, coupledto the GPS antenna to receive the data representative of GPS signals andto process these signals in order to provide, at least in oneembodiment, pseudorange information. The processor, such as a digitalprocessor, also processes communication signals received through acommunication link such that the processor typically demodulates thecommunication signals which are sent to the combined system. In thismanner, the processing of GPS signals as well as communication signalsis performed in a processor which is shared between the two functions.

In an alternative embodiment, a mobile system having a GPS receiver andan integrated communication system, such as a communication receiver,includes an antenna for receiving data representative of GPS signals, afrequency converter coupled to the antenna, a frequency synthesizercoupled to the frequency converter, an analog to digital convertercoupled to the frequency converter, and a digital processor coupled tothe frequency converter. This digital processor processes datarepresentative of GPS signals received through the antenna to determinepseudorange information based on the data representative of GPS signals.The integrated communication receiver includes a shared component whichis shared with the GPS system, such as the antenna, the frequencyconverter, the frequency synthesizer, the digital processor, a memorywhich is coupled to the digital processor or the analog to digitalconverter.

The present invention also provides a method for controlling acommunication link and processing data representative of GPS signals ina combined system. This method includes, in a typical embodiment, theprocessing of data representative of GPS signals in a processing unitand the controlling of communication signals through a communicationlink by using the processing unit to perform the controlling step,wherein the processing unit performs demodulation of communicationsignals sent to the GPS receiver.

In a typical embodiment of the present invention, the GPS operation andthe communications reception/transmission operation are performed atdifferent times, which facilitates the use of common (shared) circuitry.In addition, the signal processing operations for the GPS signals isperformed typically in a programmable digital signal processing (DSP)integrated circuit using Fast Fourier Transfer algorithms together withother data compression methods. This approach provides superioracquisition time and receiver sensitivity compared with traditionalcorrelator based approaches in traditional GPS receivers. It will beappreciated that these methods for processing GPS signals are compatiblewith programmable DSP implementation on such DSP integrated circuits,and these same circuits may be used to implement communicationdemodulators based upon similar approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in whichreferences indicate similar elements and in which:

FIG. 1A is a block diagram of the major components of a mobile combinedsystem having a GPS reception system as well as a communication systemwhich can establish a communication with a basestation.

FIG. 1B shows a block diagram of a typical implementation for the RF toIF converter and frequency synthesizer of FIG. 1A.

FIG. 2 illustrates a flow chart which indicates various processing stepsin one embodiment of the present invention.

FIG. 3 shows a flow chart of the major operations performed by the DSPprocessor in accordance with one embodiment of the present invention.

FIGS. 4A, 4B, 4C, 4D and 4E illustrate the signal processing waveformsat various stages of processing of GPS signals according to the methodsof one embodiment of the present invention.

FIG. 5A illustrates a basestation system in one embodiment of thepresent invention.

FIG. 5B illustrates a basestation in an alternative embodiment of thepresent invention.

FIG. 6 illustrates a GPS mobile unit having, according to one aspect ofthe present invention, a local oscillator correction or calibration.

FIG. 7A is a depiction of a combined GPS and communication systemaccording to an alternative embodiment of the present invention.

FIG. 7B shows a combined GPS and communication system according toanother embodiment of the present invention.

FIG. 8 is a flowchart which illustrates various steps involved inmanaging power consumption in a combined GPS and communication systemaccording to one embodiment of the invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to methods and devices for performing dualfunctions in a combined system using shared circuitry where thefunctions include (a) position determination via GPS signal processingand (b) communication to and from other locations via a radio frequencycommunication link. By sharing circuitry for these functions such thatthe shared circuitry performs at least a portion of both of thesefunctions, the system may have reduced power dissipation and reducedsize and cost. Moreover, the complexity related to interfacing suchsystems may be reduced.

An example of a combined GPS and communication system having sharedcircuitry is shown in FIG. 1A. A combined mobile unit 100 includescircuitry for performing the functions required for processing GPSsignals as well as the functions required for processing communicationsignals received through a communication link. The communication link,such as communication link 14a, is typically a radio frequencycommunication link to another component, such as a basestation 17 havinga communication antenna 14. The combined mobile unit 100 includes a GPSantenna 1 and a communication antenna 2 which are coupled to a switch 6through preselect filters 3 and 4 as shown in FIG. 1A. Depending uponwhether a GPS signal or communication signal is beingtransmitted/received, one of the two antennas 1 or 2 is selected via theswitch 6. Separate preselection filters 3 and 4 are used to removeinterference outside of the particular signal band. One such bandcorresponds to that of the GPS signals from antenna 1, and the otherband to the band where the communication signals reside. In some specialcases, it is possible to use a single antenna if the GPS signals andcommunication signals are closely spaced in frequency. It will beappreciated that the switch 6 selects the signal from the preselectfilter 3 for input to the RF to IF converter 7 when the GPS signals arebeing collected in accordance with the present invention. Whencommunication signals from the communication link 14a are beingcollected for demodulation, the switch 6 selects the signal from thepreselect filter 4 for input to the converter 7.

The output of the switch 6 is coupled to an input of the radio frequency(RF) to intermediate frequency (IF) converter 7. This converter 7translates the signal to a suitable intermediate frequency, for example70 MHz. It then provides a further translation to a lower intermediatefrequency, for example 1 MHz. Each converter within the RF to IFconverter 7 typically consists of an amplifier and a mixer, as shown inFIG. 1B. The components of the first converter are normally sufficientlywide band to encompass a wide frequency range (for example 800 to 2000MHz) and for most cases are sufficiently broad band to handle thefrequency ranges spanned by the GPS signals and the most importantcommunication signals.

The output of the RF to IF converter 7 is coupled to the input of ananalog to digital (A/D) converter 8, which digitizes the output signalsfrom the Rf to IF converter 7. In some implementations, the RF to IFconverter 7 provides a pair of outputs that are in phase quadrature; insuch cases, two AND Converters may be employed. The output from the A/DConverter 8 is coupled to an input of the digital snapshot memory 9which can store the record of data to be processed. In some cases thismemory 9 may be bypassed and the data sent directly to the processorcomponent 10 (which may be a DSP chip as shown or a set of digitalprocessing chips) if the data rate output from the AND Converter 8 issufficiently low. The snapshot memory 9 is typically used in processingthe GPS signals which are typically stored in the memory 9. The snapshotmemory 9 is also normally employed for communication signals that arepacketized--that is, signals consisting of bursts of data bits followedby long periods of inactivity. This is the primary form of communicationsignaling envisioned to be used with the present invention. However,continuous signaling, such as many cellular type signals, may beprocessed in a continuous manner by the processor 10.

The memory 9 is bidirectionally coupled to the processor 10 in a typicalembodiment in order for the processor to read and write data to thememory 9. In one embodiment the memory 9 may be conventional dual portmemory, having one input port coupled to receive output from the A/Dconverter 8 and another input port coupled to receive data fromprocessor 10.

It will be appreciated that the processing component 10 receivescommunication signals sent through the communication link 14a byconverting the communication signals in converter 7 and digitizing thosesignals through converter 8 and storing the signals in memory 9 orprocessing them directly. In this fashion, the processor. 10 demodulatesthe communication signal in order to determine the commands in thecommunication signal or other data (e.g. Doppler data or datarepresentative of ephemeris of satellites in view) in the communicationsignal.

When a transmission is required through the communication link, theprocessor 10 generates the data to be transmitted and baseband digitalsamples of the signal. It then uses this data to modulate a carriersignal using a modulator circuit 11. Such modulation is often a digitaltype, such as frequency shift keying or phase shift keying. Analogmodulation, such as frequency modulation, may also be used. The carrierfrequency at which the modulation is performed may or may not be at thefinal RF frequency of the communication signal; if it is at anintermediate frequency (IF), then an additional IF to RF converter 12 isemployed to translate the signal to a final RF frequency for thecommunication signal. A power amplifier 13 boosts the signal level, andthis boosted signal is then fed to the communication antenna 2 through atransmit/receive (T/R) switch 5 whose purpose is to isolate thesensitive receiver stage from the strong signal levels output from thepower amplifier 13. In this manner, a communication signal containingdata representative of position information (e.g. pseudoranges tovarious satellites or a latitude and longitude of the combined mobileunit 100) is transmitted to a basestation, such as basestation 17through communication link 14a.

It may be appreciated that, at least in one embodiment, the samefrequency synthesizer is used to produce the local oscillators for alloperational modes; those modes include the reception of datarepresentative of GPS signals, the reception of communication signalsfrom the communication link 14a and the transmission of communicationsignals to the communication link 14a. It should be also be noted thatthe RF to IF converter 7, the analog to digital converter 8, the digitalsnapshot memory 9 and the processor chip 10 are common to alloperational modes at least in one embodiment of the present invention.Of course, other peripheral circuitry such as power supplies, wouldnormally be common to all such modes.

It will also be appreciated that, according to one embodiment of thepresent invention, a power management circuit may be implemented usingpower management algorithms stored in the memory 19 . These algorithmscontrol the processor 10 which in turn controls the transmit powercontrol 18. The transmit power control 18 provides a controlled powersignal for the power amplifier 13, the converter 12 and the modulator 11such that after transmission of a communication signal, the transmitpower control unit 18 may cause modulator 11, converter 12 and amplifier13 to enter a reduced power state. These components typically remain inthis reduced power state until a further transmission through thecommunication link 14a is required. A typical example of this embodimentis a two-way pager system where the mobile unit 100 performs thefunctions of a two-way receiver and transmitter, and the transmitter isturned off (or otherwise consumes reduced power) when the transmitter isnot transmitting.

FIG. 1B provides some additional details of the RF to IF converter 7 andits relationship to the frequency synthesizer 16, both of which areshown in FIG. 1B. A dual frequency synthesizer 42 as shown in FIG. 1B,is commonly available and is used to provide tunable local oscillators(L.O's). These may be tuned to accommodate the different RF frequenciesfor the different operational modes. The amplifier 30 in the firstconverter 32 receives the output from the switch 6 and amplifies thatoutput to provide an input to the mixer 31 which also receives an inputfrom the oscillator 41. The output from the mixer 31 is provided to aninput of an amplifier 33, the output of which is coupled to the input ofa bandpass filter (BFP) 34. The output from this filter 34 is coupled tothe input of the second converter 37, which also includes an amplifier35 and a mixer 36. The output from the mixer 36 is provided to anautomatic gain control 38 which automatically controls the gain of thesignal and provides an output to a low pass filter 39, the output ofwhich is amplified by an amplifier 40 and provided as the output of theconverter 7 to the input of the analog to digital converter 8. Localoscillators 41 and 44 provide the tuned frequencies for the twoconverters 32 and 37 in order to perform demodulation in the receptionoperational modes of the invention. These L.O.'s 41 and 44 also providethe tuned frequencies for the modulator 11 and the converter 12 in thetransmission mode of the present invention. It will be appreciated thatthe use of a general purpose DSP integrated circuit chip (or severalchips in a chip set) to process common communication signals is wellknown to those skilled in the art. As examples of such processing, onecan refer to the data sheets of the parts TMS320CM545 and TMS320C546from Texas Instruments of Dallas, Tex.; these data sheets describe theprocessing of GSM signals that are utilized in the European digitalcellular networks.

When receiving a communications signal (e.g. from a basestation 17), theprocessor 10 causes the frequency synthesizer 16 to adjust its firstlocal oscillator 41 to provide an output frequency which is a valueeither above or below the carrier frequency of the communications signalby an amount equal to the center frequency of the SAW filter 34. Whenreceiving a GPS signal (e.g. from a GPS satellite) the processor 10causes the local oscillator 41 to provide an output frequency which is avalue either above or below the carrier frequency of the GPS signal(1575.42 MHz for the U.S. GPS system) by an amount equal to the centerfrequency of the SAW filter 34. In most situations, the second L.O. 44will be tuned to the same frequency in both cases and thus the samefinal IF will be produced in both cases. It will be appreciated that, ina typical embodiment, the processor 10 will provide the control signals(e.g. via interconnect 14 shown in FIG. 1A) to the frequency synthesizer16 in order to tune the oscillators (e.g. L.O. 41) for either GPS signalreception or communication signal reception. Similarly, the processor 10will provide the control signals to the frequency synthesizer 16 whenlocal oscillator signals are required for transmission of communicationsignals through modulator 11 and, optionally, converter 12.

The flow chart of FIG. 2 shows an example of how the apparatus of FIG.1A may be utilized in a typical operational scenario. In this situation,the receiver begins in a communications receiving mode such thatcommunication signals from the communication link 14a are beingmonitored. Therefore, in step 20 the processor 10 tunes the converter 7to the communication system access channel. This is a channel (typicalof cellular networks) that broadcasts addresses of users for which thereare messages and assigns such users to other channels in which they maycommunicate. If the receiver is being addressed in step 21, then theprocessor 10 tunes to the specified channel and, during its acquisition,measures the carrier frequency accurately in step 22. This is done inorder to calibrate the local oscillator in the mobile system 100. Ifthere is a command to determine position, which may be referred to as aGPS command as indicated in step 23, then this measurement of thecarrier frequency allows the GPS receiver to compensate for localoscillator errors thereby speeding the acquisition of GPS signals. Thereceiver then enters the GPS mode, and Processor 10 retunes the receiverto the GPS band and collects and processes the GPS signals in step 24.It may use information supplied via the communication channel during theprior operation; such information may include satellite Dopplerinformation, differential GPS data, data representative of satelliteephemeris for satellites in view, etc. In step 25, the processor 10calculates position information from the GPS signals; typically,pseudoranges to the satellites in view are calculated at this time.Further details in connection with these procedures are described in thethree above referenced patent applications filed by Norman F. Krasner onMar. 8, 1996, and these three patent applications are herebyincorporated by reference. Following the position location operation instep 25, the system enters transmit mode 26, where it transmits positioninformation to the communication link 14a. Depending upon thecommunication system and the time to perform the GPS positioncalculations, the same or different channel may be utilized as wasemployed during the reception of a message from the communication link14a. If a different channel is employed, then the channel accessprocedure used during reception may be again utilized.

It will be appreciated by those in the art that the forgoing descriptionis a typical flow according to one operational scenario. Othervariations on this scenario may be practiced according to the invention.For example, a multiplicity of GPS measurements can be made betweenreceptions or transmissions over the communication link; alternatively,a large number of communication messages may be passed back and forthover the communication link, with only occasional times allocated forprocessing of GPS signals.

The manner in which the processor 10 is used to process GPS signals forposition location is now explained.

Details of the signal processing performed in the DSP 10 may beunderstood with the aid of the flow chart of FIG. 3 and the pictorial ofFIGS. 4A, 4B, 4C, 4D and 4E. It will be apparent to those skilled in theart that the machine code, or other suitable code, for performing thesignal processing to be described is stored in memory 19. Suitable codefor controlling reception and transmission of communication signalsthrough a communication link (such as, for example, a two-way pagersystem) may also be stored in program memory 19. Other non-volatilestorage devices could also be used. The objective of the GPS processingis to determine the timing of the received waveform with respect to alocally generated waveform. Furthermore, in order to achieve highsensitivity, a very long portion of such a waveform, typically 100milliseconds to 1 second, is processed.

In order to understand the processing, one first notes that eachreceived GPS signal (C/A mode) is constructed from a high rate (1 MHz)repetitive pseudorandom (PN) pattern of 1023 symbols, commonly called"chips." These "chips" resemble the waveform shown in FIG. 4A. Furtherimposed on this pattern is low rate data, transmitted from the satelliteat 50 baud. All of this data is received at a very low signal-to-noiseratio as measured in a 2 MHz bandwidth. If the carrier frequency and alldata rates were known to great precision, and no data were present, thenthe signal-to-noise ratio could be greatly improved, and the datagreatly reduced, by adding to one another successive frames. Forexample; there are 1000 PN frames over a period of 1 second. The firstsuch frame could be coherently added to the next frame, the result addedto the third frame, etc. The result would be a signal having a durationof 1023 chips. The phasing of this sequence could then be compared to alocal reference sequence to determine the relative timing between thetwo, thus establishing the so-called pseudorange.

The above process is typically carried out separately for each satellitein view from the same set of stored received data in the snapshot memory9, since, in general, the GPS signals from different satellites havedifferent Doppler frequencies and the PN patterns differ from oneanother.

The above process is made difficult by the fact that the carrierfrequency may be unknown by in excess of 5 kHz due to signal Doppleruncertainty and by an additional amount due to receiver local oscillatoruncertainty. These Doppler uncertainties are removed in one embodimentof the present invention by transmission of such information from abasestation 17 which simultaneously monitors all GPS signals from inview satellites. Thus, Doppler search is avoided at the remote unit 100.The focal oscillator uncertainty is also greatly reduced (to perhaps 50Hz) by the AFC operation performed using the basestation to mobile unitcommunication signal (and the precision carrier frequency signal), asillustrated in FIG. 6.

The presence of 50 baud data superimposed on the GPS signal still limitsthe coherent summation of PN frames beyond a period of 20 msec. That is,at most 20 frames may be coherently added before data sign inversionsprevent further processing gain. Additional processing gain may beachieved through matched filtering and summation of the magnitudes (orsquares of magnitudes) of the frames, as detailed in the followingparagraphs.

The flow chart of FIG. 3 begins at step 101 with a command from thebasestation 17 to initialize a GPS processing operation (termed a "FixCommand" in FIG. 3). This command includes (in one embodiment) sending,over a communication link 14a, the Doppler shifts for each satellite inview and an Identification of those satellites. At step 102, the remoteunit 100 computes its local oscillator drift by frequency locking to thesignal transmitted from the basestation 17. An alternative would be toutilize a very good quality temperature compensated crystal oscillatorin the remote unit. For example, digitally controlled TCXOs, so-calledDCXOs, currently can achieve accuracy of about 0.1 parts per million, oran error of about 150 Hz for the L1 GPS signal.

At step 104 the remote unit's processor 10 collects a snapshot of dataof duration K PN frames of the C/A code, where K is typically 100 to1000 (corresponding to 100 msec to 1 second time duration). When asufficient amount of data has been collected, the processor 10 mayreduce power consumed by the RF to IF converter 7 and the A/D converters8 by placing these components in a reduced power state for at least aperiod of time (e.g. a short predetermined period of time). After thisperiod of time, full power is typically provided again to thesecomponents in order to detect whether communication signals are beingtransmitted to the remote/mobile unit 100. This cycle of reduced andfull power may be repeated as shown in FIG. 8 which is discussed below.

The pseudorange of each satellite is computed in turn as follows. First,at step 106 for the given GPS satellite signal to be processed, thecorresponding pseudorandom code (PN) is retrieved from memory 19. Asdiscussed shortly, the preferred PN storage format is actually theFourier transform of this PN code, sampled at a rate of 2048 samples perthe 1023 PN bits.

The data in snapshot memory 9 is processed in blocks of N consecutive PNframes, that is blocks of 2048N complex samples (N is an integertypically in the range 5 to 10). Similar operations are performed oneach block as shown in the bottom loop (steps 108-124) of FIG. 3. Thatis, this loop is performed a total of K/N times for each GPS signal tobe processed.

At step 108 the 2048N data words of the block are multiplied by acomplex exponential that removes the effects of Doppler on the signalcarrier, as well as the effects of drifting of the receiver's localoscillator. To illustrate, suppose the Doppler frequency transmittedfrom the basestation 17 plus local oscillator offsets corresponded tof_(e) Hz. Then the premultiplication of the data would take the form ofthe function e^(-j2)πf_(e) ^(nT), n=[0, 1, 2, . . . , 2048N-1]+(B-1)×2048N, where T=1/2.048 MHz is the sampling period, and theblock number B ranges from 1 to K/N.

Next, at step 110, the adjacent groups of N (typically 10) frames ofdata within the block are coherently added to one another. That is,samples 0, 2048, 4096, . . . 2048(N-1)-1 are added together, then 1,2049, 4097, . . . 2048(N-1) are added together, etc. At this point theblock contains only 2048 complex samples. An example of the waveformproduced by such a summing operation is illustrated in FIG. 4B for thecase of 4 PN frames. This summing operation may be considered apreprocessing operation which precedes the fast convolution operations.

Next, at steps 112-118, each of the averaged frames undergoes a matchedfiltering operation, whose purpose is to determine the relative timingbetween the received PN code contained within the block of data and alocally generated PN reference signal. Simultaneously, the effects ofDoppler on the sampling times is also compensated for. These operationsare greatly speeded, in one embodiment, by the use of fast convolutionoperations such as Fast Fourier Transform algorithms used in a manner toperform circular convolution, as presently described.

In order to simplify discussion, the above mentioned Dopplercompensation is initially neglected.

The basic operation to be performed is a comparison of the data in theblock being processed (2048 complex samples) to a similar reference PNblock stored locally. The comparison is actually done by (complex)multiplying each element of the data block by the corresponding elementof the reference and summing the results. This comparison is termed a"correlation." However, an individual correlation is only done for oneparticular starting time of the data block, whereas there are 2048possible positions that might provide a better match. The set of allcorrelation operations for all possible starting positions is termed a"matched filtering" operation. The full matched filtering operation isrequired in a preferred embodiment.

The other times of the PN block can be tested by circularly shifting thePN reference and reperforming the same operation. That is, if the PNcode is denoted p(0) p(1) . . . p(2047), then a circular shift by onesample is p(1) p(2) . . . p(2047) p(0). This modified sequence tests todetermine if the data block contains a PN signal beginning with samplep(1). Similarly the data block may begin with samples p(2), p(3), etc.,and each may be tested by circularly shifting the reference PN andreperforming the tests. It should be apparent that a complete set oftests would require 2048×2048=4,194,304 operations, each requiring acomplex multiplication and addition.

A more efficient, mathematically equivalent method may be employed,utilizing the Fast Fourier Transform (FFT), which only requiresapproximately 12×2048 complex multiplications and twice the number ofadditions. In this method, the FFT is taken for the data block, at step112, and for the PN block. The FFT of the data block is multiplied bythe complex conjugate of the FFT of the reference, at step 114, and theresults are inverse Fourier transformed at step 118. The resulting dataso gotten is of length 2048 and contains the set of correlations of thedata block and the PN block for all possible positions. Each forward orinverse FFT operation requires P/2 log₂ P operations, where P is thesize of the data being transformed (assuming a radix-2 FFT algorithm isemployed). For the case of interest, B=2048, so that each FFT requires11×1024 complex multiplications. However, if the FFT of the PN sequenceis prestored in memory 19, as in a preferred embodiment, then its FFTneed not be computed during the filtering process. The total number ofcomplex multiplies for the forward FFT, inverse FFT and the product ofthe FFTs is thus (2×11+2)×1024=24576, which is a savings of a factor of171 over direct correlation. FIG. 4C illustrates the waveform producedby this matched filtering operation.

The preferred method of the current invention utilizes a sample ratesuch that 2048 samples of data were taken over the PN period of 1023chips. This allows the use of FFT algorithms of length 2048. It is knownthat FFT algorithms that are a power of 2, or 4, are normally much moreefficient than those of other sizes (and 2048=2¹¹). Hence the samplingrate so chosen significantly improves the processing speed. It ispreferable that the number of samples of the FFT equal the number ofsamples for one PN frame so that proper circular convolution may beachieved. That is, this condition allows the test of the data blockagainst all circularly shifted versions of the PN code, as discussedabove. A set of alternative methods, known in the art as "overlap save"or "overlap add" convolution may be utilized if the FFT size is chosento span a number of samples different from that of one PN frame length.These approaches require approximately twice the number of computationsas described above for the preferred implementation.

It should be apparent to one skilled in the art how the above processmay be modified by utilizing a variety of FFT algorithms of varyingsizes together with a variety of sample rates to provide fastconvolution operations. In addition, a set of fast convolutionalgorithms exist which also have the property that the number ofcomputations required are proportional to B log₂ B rather than B² as isrequired in straightforward correlation. Many of these algorithms areenumerated in standard references, for example, H. J. Nussbaumer, "FastFourier Transform and Convolution Algorithms," New York,Springer-Verlag, C1982. Important examples of such algorithms are theAgarwal-Cooley Algorithm, the split nesting algorithm, recursivepolynomial nesting algorithm, and the Winograd-Fourier algorithm, thefirst three of which are used to perform convolution and the latter usedto perform a Fourier transform. These algorithms may be employed insubstitution of the preferred method presented above.

The method of time Doppler compensation employed at step 116 is nowexplained. In the preferred implementation, the sample rate utilized maynot correspond exactly to 2048 samples per PN frame due to Dopplereffects on the received GPS signal as well as local oscillatorinstabilities. For example, it is known that the Doppler shift cancontribute a delay error of ±2700 nsec/sec. In order to compensate forthis effect, the blocks of data processed in the above description needto be time shifted to compensate for this error. As an example, if theblock size processed corresponds to 5 PN frames (5 msec), then the timeshift from one block to another could be as much as ±13.5 nsec. Smallertime shifts result from local oscillator instability. These shifts maybe compensated for by time shifting the successive blocks of data bymultiples of the time shift required by a single block. That is, if theDoppler time shift per block is d, then the blocks are time shifted bynd, n=0, 1, 2, . . . .

In general these time shifts are fractions of a sample. Performing theseoperations directly using digital signal processing methods involves theuse of nonintegral signal interpolation methods and results in a highcomputation burden. An alternative approach, that is a preferred methodof the present invention, is to incorporate the processing within thefast Fourier transform functions. It is well-known that a time shift ofd seconds is equivalent to multiplying the Fourier Transform of afunction by e^(-j2)πfd, where f is the frequency variable. Thus, thetime shift may be accomplished by multiplying the FFT of the data blockby e^(-j2)πnd/T f for n=0, 1, 2, . . . , 1023 and by e^(-j2)π(n-2048)d/Tf for n=1024, 1025, . . . , 2047, where T_(f) is the PN frame duration(1 millisecond). This compensation adds only about 8% to the processingtime associated with the FFT processing. The compensation is broken intotwo halves in order to guarantee continuity of phase compensation across0 Hz.

After the matched filtering operation is complete, the magnitudes, ormagnitudes-squared, of the complex numbers of the block are computed atstep 120. Either choice will work nearly as well. This operation removeseffects of 50 Hz data phase reversals (as shown in FIG. 4D) and lowfrequency carrier errors that remain. The block of 2048 samples is thenadded to the sum of the previous blocks processed at step 122. Step 122may be considered a post processing operation which follows the fastconvolution operation provided by steps 112-118. This continues untilall K/N blocks are processed, as shown by the decision block at step124, at which time there remains one block of 2048 samples, from which apseudorange is calculated. FIG. 4E illustrates the resulting waveformafter the summing operation.

Pseudorange determination occurs at step 126. A peak is searched forabove a locally computed noise level. If such a peak is found, its timeof occurrence relative to the beginning of the block represents thepseudorange associated with the particular PN code and the associatedGPS satellite.

An interpolation routine is utilized at step 126 to find the location ofthe peak to an accuracy much greater than that associated with thesample rate (2.048 MHz). The interpolation routine depends upon theprior bandpass filtering used in the RF/IF portion of the remote unit100. A good quality filter will result in a peak having a nearlytriangular shape with the width of the base equal to 4 samples. Underthis condition, following subtraction of an average amplitude (to removea DC baseline), the largest two amplitudes may be used to determine thepeak position more precisely. Suppose these amplitudes are denoted A_(p)and A_(p+1), where A_(p) ≧A_(p+1), without loss of generality, and p isthe index of the peak amplitude. Then the position of the peak relativeto that corresponding to A_(p) may be provided by the formula: peaklocation=p+A_(p) /(A_(p) +A_(p+1)). For example if A_(p) =A_(p+1), thenthe peak location is found to be p+0.5, that is, halfway between theindices of the two samples. In some situations the bandpass filteringmay round the peak and a three point polynomial interpolation may bemore suitable.

In the preceding processing, a local noise reference used inthresholding, may be computed by averaging all the data in the finalaveraged block, after removing the several largest such peaks.

Once the pseudorange is found, the processing continues at step 128 in asimilar manner for the next satellite in view, unless all suchsatellites have been processed. Upon completion of the processing forall such satellites, the process continues at step 130 where thepseudorange data is transmitted to the basestation 17 over acommunication link 14a. The final position calculation of the remoteunit 100 is performed by the basestation in an embodiment of theinvention where the basestation computes a latitude and longitude ratherthan the mobile unit 100. Finally, at step 132, at least some of thecircuitry of the remote 100 (e.g. modulator 11, converter 12 andamplifier 13) is placed in a low power state, awaiting a new command toperform another positioning operation.

A summary of the signal processing described above and shown in FIG. 3will now be provided. The GPS signals from one or more in view GPSsatellites are received at the remote GPS unit using GPS antenna 1.These signals are digitized and stored in a buffer in the remote GPSunit. After storing these signals, a processor performs preprocessing,fast convolution processing, and post processing operations. Theseprocessing operations involve:

a) breaking the stored data into a series of contiguous blocks whosedurations are equal to a multiple of the frame period of thepseudorandom (PN) codes contained within the GPS signals.

b) for each block performing a preprocessing step which creates acompressed block of data with length equal to the duration of apseudorandom code period by coherently adding together successivesubblocks of data, the subblocks having a duration equal to one PNframe; this addition step will mean that the corresponding samplenumbers of each of the subblocks are added to one another.

c) for each compressed block, performing a matched filtering operation,which utilizes fast convolution techniques, to determine the relativetiming between the received PN code contained within the block of dataand a locally generated PN reference signal (e.g. the pseudorandomsequence of the GPS satellite being processed).

d) determining a pseudorange by performing a magnitude-squared operationon the products created from said matched filtering operation and postprocessing this by combining the magnitude-squared data for all blocksinto a single block of data by adding together the blocks ofmagnitude-squared data to produce a peak.

and e) finding the location of the peak of said single block of data tohigh precision using digital interpolation methods, where the locationis the distance from the beginning of the data block to the said peak,and the location represents a pseudorange to a GPS satellitecorresponding to the pseudorandom sequence being processed.

Typically, the fast convolution technique used in processing thebuffered GPS signals is a Fast Fourier Transform (FFT) and the result ofthe convolution is produced by computing the product of the forwardtransform of the compressed block and a prestored representation of theforward transform of the pseudorandom sequence to produce a first resultand then performing an inverse transformation of the first result torecover the result. Also, the effects the Doppler induced time delaysand local oscillator induced time errors are compensated for on eachcompressed block of data by inserting between the forward and inverseFast Fourier Transform operations, the multiplication of the forward FFTof the compressed blocks by a complex exponential whose phase versussample number is adjusted to correspond to the delay compensationrequired for the block.

In the foregoing embodiment the processing of GPS signals from eachsatellite occurs sequentially over time, rather than in parallel. In analternative embodiment, the GPS signals from all in view satellites maybe processed together in a parallel fashion in time.

It is assumed here that the basestation 17 has a common view of allsatellites of interest and that it is sufficiently close in range toremote unit 100 in order to avoid ambiguities associated with therepetition period of the C/A PN code. A range of 90 miles will satisfythis criteria. The basestation 17 is also assumed to have a GPSreceiver, and a good geographical location such that all satellites inview are continuously tracked to high precision.

While several described embodiments of the basestation 17 show the useof a data processing component, such as a computer at the basestation inorder to compute position information such as a latitude and a longitudefor the mobile GPS unit, it will be appreciated that each basestation 17may merely relay the information received, such as pseudoranges from amobile GPS unit, to a central location or several central locationswhich actually perform the computation of latitude and longitude. Inthis manner the cost and complexity of these relaying basestations maybe reduced by eliminating a data processing unit and its associatedcomponents from each relaying basestation. A central location, wouldinclude receivers (e.g. telecommunication receivers) and a dataprocessing unit and associated components. Moreover, in certainembodiments, the basestation may be virtual in that it may be asatellite which transmits Doppler information to remote units, therebyemulating a basestation in a transmission cell.

FIGS. 5A and 5B show two embodiments of a basestation according to thepresent invention. In the basestation shown in FIG. 5A, a GPS receiver501 receives GPS signals through a GPS antenna 501a. The GPS receiver501, which may be a conventional GPS receiver, provides a timedreference signal which typically is timed relative to GPS signals andalso provides Doppler information relative to the satellites in view.This GPS receiver 501 is coupled to a disciplined local oscillator 505which receives the time reference signal 510 and phase locks itself tothis reference. This disciplined local oscillator 505 has an outputwhich is provided to a modulator 506. The modulator 506 also receivesDoppler data information signals for each satellite in view of the GPSmobile unit and/or other satellite data information signals (e.g. datarepresentative of satellite ephemeris) via interconnect 511. Themodulator 506 modulates the Doppler and/or other satellite datainformation onto the local oscillator signal received from thediscipline local oscillator 505 in order to provide a modulated signal513 to the transmitter 503. The transmitter 503 is coupled to the dataprocessing unit 502 via interconnect 514 such that the data processingunit may control the operation of the transmitter 503 in order to causethe transmission of satellite data information, such as the Dopplerinformation to a GPS mobile unit (e.g. remote unit 100 having sharedcircuitry) via the transmitter's antenna 503a. In this manner, a GPSmobile unit may receive the Doppler information, the source of which isthe GPS receiver 501 and may also receive a high precision localoscillator carrier signal which may be used to calibrate the localoscillator in the GPS mobile unit as shown in FIG. 6.

The basestation as shown in FIG. 5A also includes a receiver 504 whichis coupled to receive communication signals from the remote unit 100 viaa communication antenna 504a. It will be appreciated that the antenna504a may be the same antenna as the transmitters antenna 503a such thata single antenna serves both the transmitter and the receiver in theconventional fashion. The receiver 504 is coupled to the data processingunit 502 which may be a conventional computer system. The processingunit 502 may also include an interconnect 512 to receive the Dopplerand/or other satellite data information from the GPS receiver 511. Thisinformation may be utilized in processing the pseudorange information orother information received from the mobile unit 100 via the receiver504. This data processing unit 502 is coupled to a display device 508,which may be a conventional CRT. The data processing unit 502 is alsocoupled to a mass storage device 507 which includes GIS (GeographicalInformation System) software (e.g. Atlas GIS from Strategic Mapping,Inc. of Santa Clara, Calif.) which is used to display maps on thedisplay 508. Using the display maps, the position of the mobile GPS unit100 may be indicated on the display relative to a displayed map.

An alternative basestation shown in FIG. 5B includes many of the samecomponents shown in FIG. 5A. However, rather than obtaining Dopplerand/or other satellite data information from a GPS receiver, thebasestation of FIG. 5B includes a source of Doppler and/or othersatellite data information 552 which is obtained from atelecommunication link or a radio link in a conventional matter. ThisDoppler and/or satellite information is conveyed over an interconnect553 to the modulator 506. The other input to the modulator 506 shown inFIG. 5B is the oscillator output signal from a reference quality localoscillator 551 such as a cesium standard local oscillator. Thisreference local oscillator 551 provides a precision carrier frequencyonto which is modulated the Doppler and/or other satellite datainformation which is then transmitted via transmitter 503 to the mobileGPS unit.

FIG. 6 shows an embodiment of a GPS mobile unit of the present inventionwhich utilizes the precision carrier frequency signal received throughthe communication channel antenna 601 which is similar to the antenna 2shown in FIG. 1A. Similarly, it will be appreciated that GPS antenna 613may be the same as antenna 1 in FIG. 1A, and converter 614, A/Dconverter 616, memory 618 and DSP component 620 represent respectivelyconverter 7, A/D converter 8, memory 9, and DSP component 10 in FIG. 1A.The frequency synthesizer 609 and local oscillator 606 representrespectively synthesizer 42 and frequency reference 43 shown in FIG. 1B.In this embodiment, during reception of communication signals, the DSPcomponent 620 computes the tuning error from the precision carrierfrequency signal and sends tuning corrections 610 to the frequencysynthesizer 609. These tuning corrections may then be used to determinethe initial tuning error, and hence the error in the local oscillatorsignal 607, assuming that the received communications signal has a verystable carrier frequency. The local oscillator error may then becompensated during a subsequent GPS reception operation by offsettingthe frequency synthesizer 609's tuning frequency by an amount thatnegates the effect of the local oscillator error from local oscillator606.

FIGS. 7A and 7B show alternative embodiments of the present inventionwhich utilize shared components. As shown in FIG. 7A, the processor 421(e.g. a DSP IC) is shared between two separate receiver sections. Inparticular, the converter 407, antenna 401, and converter 411 form theGPS receiver section, and the converter 413, antenna 403 and theconverter 416 provide the communication receiver section. The outputsignals from these two receivers are selected by the switch 417 undercontrol of the processor 421 through the select line 418 of the switch417. The output from this switch 417 is coupled to an input of thedigital memory 419 which is coupled to the processor 421 by abi-directional bus. A memory 420 stores computer programs and data whichcontrol the operations of the processor 10, and this memory is coupledto the processor 10. The processor controls frequency synthesizer 424through the control line 423. The frequency synthesizer 424 providesvarious local oscillator signals 409 and 415 for use by the converters407 and 413, respectively and also provides local oscillator signals 426to the modulator 425 and the converter 427. When the processor desiresto transmit a message through the communication link 405 viacommunication antenna 403, the processor sends data to the modulator 425and the modulated data is then converted in the converter 427 andamplified by the power amplifier 429. The operation of the modulator 425and the converter 427 and the amplifier 429 is similar to that describedabove for modulator 11, converter 12 and amplifier 13 of FIG. 1A. FIG.7A may be most appropriate for use with certain cellular communicationssystems, such as the analog North American AMPS system, in which atelephone conversation and a GPS operation may be occurringconcurrently. In this situation it may be Impossible to share theconverters; however, the frequency synthesizer, digital memory anddigital processor may be shared in order to save, size, cost and power.In this case the frequency synthesizer is one that can provide amultiplicity of local oscillators from a single reference source suchthat local oscillator signals are simultaneously provided for severalconcurrent operations (e.g. telephone signal reception and transmissionand GPS signal reception).

The processor 421 controls power consumption in the combined GPS andcommunication system shown in FIG. 7A by, in one embodiment, reducingpower consumed by the transmitter section which includes the modulator425, IF to RF converter 427 and the power amplifier 429. This powerreduction is achieved by the transmit power control 431 which providescontrolled power to this transmitter section through interconnects 432and 434; the processor 421, through control signals on interconnect 431a, instructs the transmit power control 431 to provide full power orreduced power in accordance with the operational mode of thetransmitter. In a similar manner, the processor 421, under control of apower management program stored in the memory 420 (which may be an EPROMor EEPROM memory), may place the GPS receiver section in a reduced powerconsumption mode when GPS signals are not being received. For example,power may not be provided, in the reduced power consumption mode, toconverters 407 and 411.

FIG. 7B shows another embodiment accordingly to the present inventionwherein the receiver portions are shared but the processing portions arenot. The receiver portion shown in FIG. 7B is similar to the receiverportion shown in FIG. 1A, where the converter 459, converter 463 andfrequency synthesizer 461 provide the essential components of both GPSand communication signal receiver sections and are shared between bothreceiver sections. The transmitter portion shown in FIG. 7B is alsosimilar to the transmitter portion shown in FIG. 1A, and includes themodulator 479, the IF to RF converter 481, the power amplifier 483, thetransmit power control 485 and the switch 487. The frequency synthesizer461 also provides local oscillator signals to the modulator 479 and theIF to RF converter 481 in the transmitter portion shown in FIG. 7B.However, as shown in FIG. 7B, there are two separate processors forprocessing the two functions of the combined system. Communicationprocessor 473 controls processing (e.g. demodulation and modulation) ofthe communication signals while the GPS processor 475 processes the GPSsignals; the results (e.g. a position information) of the processing ofGPS signals are communicated through the shared memory 471 to thecommunication processor 473 which then communicates a positioninformation through interconnect 477 to the transmitter portion whichcomprises modulator 479, converter 481, power amplifier 483 and thetransmit/receiver switch 487. The processor 473 in the embodiment shownin FIG. 7B controls the switching of the frequency synthesizer 461between the various operational modes (e.g. communication receive or GPSreceive). The processor 473 typically also controls the switching ofswitch 465 (through control line 464) so that GPS signals are stored inthe GPS memory 467 (when the shared receiver is operating in GPSreception mode) and so that communication signals are stored in thecommunication memory 469 when the shared receiver is operating in thecommunication signal reception mode.

FIG. 7B may be appropriate to systems such as two-way pagers and similarsuch systems, in which the communications reception operation and theGPS reception operation need not be concurrent. Here, most of the RFchain and the A/D converter may be shared. FIG. 7B, unlike FIG. 1A,however, provides for separate digital processors, as may be necessary,if the combined processing burden of the GPS and communicationsprocessing functions are too severe to allow completion within a desiredtime period. As with the system shown in FIG. 7A, reduction of powerconsumption may be achieved by the processor 473 causing the reductionof power consumed by the transmitter portion through the transmit powercontrol 485.

FIG. 8 illustrates a typical method according to the invention forconserving and reducing power consumption in a combined GPS andcommunication system having shared circuitry. As an example, the methodof FIG. 8 will be described for the combined system shown in FIG. 1A.This method typically operates by the processor 10 controlling powerreduction under control of a program stored in memory 19. Powerreduction is typically achieved by controlling power interconnects tovarious components. For example, the transmitter section receives powercontrolled interconnects through a transmit power control 18. Similarly,the receiver section may receive power through power controlledinterconnects (not shown) which provide power to components (e.g.converters 7 and 8) in the shared receiver section. It will beappreciated that in some applications power may be provided withoutinterruption to the reference oscillators and phase lock loops (PLL) inthe frequency synthesizer since these components require some time tostabilize after power is first provided. The method begins at step 801,where full power is provided to the communication receiver; thisreceiver includes the RF to IF converter 7 and the A/D converter 8 andthe preselect filter 4. Any communication signals received during thistime are stored in memory 9 and demodulated by processor 10.

The processor 10 determines in step 803 whether the communicationsignals during step 801 include a request to provide positioninformation of the combined system; this request is also referred to asa "fix command". If no such request is received, power to thecommunication receiver is reduced in step 805 and the processor 10, instep 807, waits a period of time (e.g. a predetermined period of time)before returning to step 801. If such a request is received, theprocessor 10 causes, in step 809, full power to be provided tocomponents of the shared GPS/communication receiver which have notalready received full power; an example of such components includes thepreselect filter 3 (which may include a low noise amplifier) which mayremain in a reduced power state while the communication signals arereceived. The processor 10, in step 815, processes any communicationdata received by the communication receiving operation. Such data mayinclude satellite Doppler information for satellites in view and anidentification of these satellites. Then in step 820, the GPS receiverof the shared GPS/Communication receiver receives the GPS signals fromthe satellites in view and digitized versions of the signals are storedin memory 9. Processor 10 then causes the power consumed by the sharedGPS/Communication receiver (e.g. converters 7 and 8) to be reduced instep 825, and in step 830, the processor 10 processes the stored GPSsignals. After the processor 10 determines a position information (e.g.pseudoranges to a plurality of satellites in view or a latitude andlongitude of the combined system), the processor 10 causes, in step 835,power to be provided to the transmitter section by instructing thetransmit power control 18 to provide full power to the modulator 11,converter 12 and power amplifier 13. Then the transmitter, in step 840transmits the position information and then, in step 845, power providedto the transmitter section is reduced. Processing then returns back tostep 801 to continue from this point.

In the foregoing specification the invention has been described withreference to specific explementary embodiment thereof. It will howeverbe evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the pending claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. A method for performing Global Positioning System(GPS) functions and cellular communication functions in an integratedcommunication system, said method comprising:converting radio frequency(RF) GPS signals and RF cellular communication signals in a shared RF tointermediate frequency (IF) converter, wherein said shared RF to IFconverter converts said RF GPS signals from a first frequency to asecond frequency which is lower than said first frequency and saidshared RF to IF converter converts said RF cellular communicationsignals from a third frequency to a fourth frequency which is lower thansaid third frequency, and wherein said shared RF to IF convertercomprises a shared frequency mixer which converts said RF GPS signals tosaid second frequency and which converts said RF cellular communicationsignals to said fourth frequency.
 2. A method as in claim 1 wherein saidshared RF to IF converter performs a final frequency conversion on bothsaid RF GPS signals and said RF cellular communication signals.
 3. Amethod as in claim 1 wherein said first frequency and said thirdfrequency are different by more than 100 MHz.
 4. A method as in claim 2wherein GPS signals at said second frequency and cellular communicationsignals at said fourth frequency are filtered or amplified in saidshared RF to IF converter after said final frequency conversion.
 5. Amethod as in claim 4 wherein said shared RF to IF converter provides ashared signal path in said shared RF to IF converter for both said RFGPS signals and said RF cellular communication signals.
 6. A method asin claim 2 further comprising:converting, in a shared analog to digital(A/D) converter, GPS signals at said second frequency, from said sharedRF to IF converter, to digitized GPS signals; converting, in said sharedA/D converter, cellular communication signals, from said shared RF to IFconverter, to digitized cellular communication signals.
 7. A method asin claim 1 wherein said shared RF to IF converter is selectably tunableto (a) convert from said first frequency to said second frequency; and(b) convert from said third frequency to said fourth frequency.
 8. Amethod as in claim 1 wherein said RF cellular communication signalscomprise at least one of a Doppler information of a GPS satellite inview of said integrated communication system and data representative ofephemeris for a GPS satellite in view of said integrated communicationsystem.
 9. An integrated communication system having a GlobalPositioning System (GPS) receiver which is coupled to a cellularcommunication system, said integrated communication system comprising:ashared radio frequency (RF) to intermediate frequency (IF) converterwhich is coupled to receive RF GPS signals and is coupled to receive RFcellular communication signals, said shared RF to IF converter having ashared frequency mixer which converts said RF GPS signals from a firstfrequency to a second frequency which is lower than said secondfrequency, and said shared frequency mixer converting said RF cellularcommunication signals from a third frequency to a fourth frequency whichis lower than said third frequency.
 10. An integrated communicationsystem as in claim 9 wherein said shared frequency mixer has a firstinput for receiving said RF cellular communication signals and said RFGPS signals and has a second input for receiving at least one referencesignal and has an output for providing IF GPS signals to said GPSreceiver and for providing IF cellular communication signals to saidcellular communication system.
 11. An integrated communication system asin claim 9 wherein said shared frequency mixer performs a finalfrequency conversion on both said RF GPS signals and said RF cellularcommunication signals.
 12. An integrated communication system as inclaim 10 further comprising:a shared analog to digital (A/D) convertercoupled to said shared RF to IF converter to receive said IF GPS signalsand to receive said IF cellular communication signal, said shared A/Dconverter converting said IF GPS signals to digitized GPS signals forprocessing in said GPS receiver and converting said IF cellularcommunication signals to digitized cellular communication signals forprocessing in said cellular communication system.
 13. An integratedcommunication system as in claim 10 wherein said shared frequency mixeris selectable tunable to (a) convert from said first frequency to saidsecond frequency and (b) convert from said third frequency to saidfourth frequency.
 14. An integrated communication system as in claim 9wherein said RF cellular communication signals comprise at least one ofa Doppler information of a GPS satellite in view of said integratedcommunication system and data representative of ephemeris for a GPSsatellite in view of said integrated communication system.
 15. Anintegrated communication system having a Global Positioning System (GPS)receiver which is coupled to a cellular communication system, saidintegrated communication system comprising:a shared analog to digital(A/D) converter coupled to receive GPS signals and to receive cellularcommunication signals, said shared A/D converter converting said GPSsignals to digitized GPS signals for processing in said GPS receiver andconverting said cellular communication signals to digitized cellularcommunication signals for processing in said communication system. 16.An integrated communication system as in claim 15 further comprising:adigital signal processor (DSP) coupled to said shared A/D converter toreceive said digitized cellular communication signals, said DSPdemodulating said digitized cellular communication signals.
 17. Anintegrated communication system as in claim 16 wherein said DSP receivessaid digital GPS signals and processes said digital GPS signals.
 18. Amethod for performing Global Positioning System (GPS) functions andcellular communication functions in an integrated communication system,said method comprising:processing GPS signals and cellular communicationsignals in a shared circuitry, said processing in said shared circuitrybeing after a final frequency conversion of said GPS signals and after afinal frequency conversion of said cellular communication signals andprior to digitization of said GPS signals and prior to digitization ofsaid cellular communication signals, said processing being performed inorder to determine the message content of said GPS signals and saidcellular communication signals.